1. Field of the Invention
The present invention relates to a sintered silicon nitride member, and to a sintered silicon nitride member in the form of a ceramic ball.
2. Description of the Related Art
Recently, silicon nitride ceramic has been applied to bearings of machine tools and hard disk drives of computers, because of its high strength and excellent wear resistance. Also, because of its corrosion resistance at high temperature, silicon nitride ceramic has been applied to bearings and sliding components used in high-temperature, corrosive environments, such as bearings and sliding components of driving units of semiconductor equipment and sliding components of automobiles, such as tappets.
In manufacture of the above-mentioned ceramic components, after precision polishing, the ceramic components are inspected for material-related defects, such as foreign matter and pores, and defects caused by polishing, such as scratches, cuts, and chips. This inspection may be carried out using a stereomicroscope or metallographic microscope. Recently, in order to cope with mass production and to reduce cost, an automatic appearance inspection machine has been employed, in some cases. Specifically, an image of a polished surface is captured, and the captured image is inspected for defects, such as foreign matter or pores on the polished surface. Such defects are identified by means of contrast of color tone or brightness between a defective portion and a background portion.
3. Problems to be Solved by the Invention
Generally, the appearance of a sintered silicon nitride body assumes a color tone of white or bright gray. Similarly, a defect such as a pore or a chip appears in a bright tone. Thus, contrast is hardly produced between such a defect and the background, resulting in poor accuracy in defect identification. Particularly, in the case of an automatic appearance inspection machine, which identifies a defect by processing data regarding the difference in color and/or lightness between background and a defective portion, low contrast therebetween is directly linked to an impairment in inspection accuracy. According to a method disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 254471/1992 and 254473/1992, a sintered body is intentionally colored black or dark gray by sintering a shaped body containing a source of carbon or by impregnating a porous body with carbon, to thereby decrease nonuniform color tone appearing on the sintered body. Background in black or dark gray facilitates identification of, for example, a pore or a chip, which appears in a relatively bright tone, but encounters difficulty in identifying foreign matter, particularly metallic foreign matter, which appears in black or blackish gray.
It is therefore an object of the present invention to provide a sintered silicon nitride member which facilitates identification of defects and foreign matter of different color tones and which is capable of enhancing inspection accuracy, as well as to provide a ceramic ball formed of the same.
The above objects of the present invention have been achieved by providing a sintered silicon nitride member formed predominantly of silicon nitride, wherein at least a surface of the sintered member has a lightness VS of 3.0 to 9.0. As used herein, the lightness VS is defined in a color specification as specified by JIS Z 8721 (published Feb. 19, 1993, Japanese Standards Association).
In the present invention, unless otherwise specified, the term xe2x80x9cpredominantxe2x80x9d used in relation to content means that the content of a substance in question is contained in an amount of not less than 50% by weight (the terms xe2x80x9cpredominantlyxe2x80x9d and xe2x80x9cmainlyxe2x80x9d have the same meaning as used herein).
In the present invention, by imparting a lightness VS of 3.0 to 9.0 to the surface of a sintered silicon nitride member, the surface, particularly when polished, of the member establishes a background color tone which facilitates identification of a defect, such as a pore, a chip, or a cut, as well as foreign matter. The background is toned differently from such a defect, thereby improving inspection accuracy of, for example, an automatic appearance inspection machine.
When the polished surface of a ceramic component is inspected for metallic foreign matter and defects, such as pores, cuts, and chips (hereinafter, these defects are generically called a xe2x80x9cpore, etc.xe2x80x9d), foreign matter or a pore, etc., is identified by means of difference in color or lightness between a base material serving as background and the foreign matter or pore, etc. Specifically, by observing with a stereomicroscope or by observing with a metallographic microscope using polarized light, metallic foreign matter appears in a color tone close to black, whereas an internal pore or a cut appears in a color tone close to white.
Herein, the meaning of color includes not only chromatic color but also achromatic color that has no chroma. Therefore, when it is acknowledged that colors are different from each other, it means that at least one of lightness, chroma and hue differs between the colors.
By imparting a lightness VS of 3.0 to 9.0 to a background portion, at a portion where metallic foreign matter or a pore, etc., is present, the background assumes a substantially neutral tint between metallic foreign matter and a pore, etc., thereby producing a distinct contrast with both metallic foreign matter and a pore, etc., which assume different color tones. Thus, metallic foreign matter or a pore, etc. can be readily identified by observing with a stereomicroscope or with a metallographic microscope using polarized light.
When the base material assumes a lightness of less than 3.0 (i.e., dark black), metallic foreign matter is hardly identified, since metallic foreign matter appears in black. When the base material assumes a lightness of greater than 9.0 (i.e., near white), a pore, etc., is hardly identified, since the pore, etc., appears in white. The above-mentioned lightness VS is preferably 4.0 to 8.5, more preferably 4.5 to 8.0. In order to enhance contrast with foreign matter or a pore, etc., the appearance of the sintered silicon nitride member assumes a chroma CS of not greater than 3.0 as defined by JIS Z 8721 (published Feb. 19, 1993, Japanese Standards Association), preferably not greater than 2.0, more preferably not greater than 1.0.
The lightness VS and the chroma CS are measured according to xe2x80x9c4.3 Measuring method for reflecting objectsxe2x80x9d of xe2x80x9c4. Spectrophotometric methodxe2x80x9d in JIS Z 8722 (published Mar. 4, 1994, Japanese Standards Association) xe2x80x9cMethods of color measurement.xe2x80x9d Among conditions a to d specified in the section 4.3, an optimum condition may be selected according to the shape of a surface to be measured. For example, when the polished surface of a ceramic ball, which will be described below, is to be measured, condition d (A specimen shall be illuminated with a bundle of rays whose beam axis angle forms an angle not exceeding 10xc2x0 referring to the normal line to the specimen surface, and the light reflected in all directions shall be collected and received. The bundle of illuminating rays shall not include a ray deviating by 5xc2x0 or more from its center line.) is preferred. As a simplified method, lightness and chroma may be determined by visual comparison with standard color chips prepared in accordance with JIS Z 8721 (1993).
Typically, ceramic balls formed of the sintered silicon nitride member of the present invention are applied to bearing balls used, for example, in machine tools and hard disk drives of computers, as well as to bearing balls used in high-temperature, corrosive environments, such as bearing balls of driving units of semiconductor equipment. This is because the ceramic balls have good corrosion resistance at high temperature. The inventive sintered silicon nitride member is also applicable to mechanical sliding components other than bearings, such as tappets used in automobiles. However, application of the present invention is not limited thereto. When the inventive sintered silicon nitride member is applied to bearing balls, the diameter thereof is about 1 mm to 30 mm. When the invented sintered silicon nitride member is applied to a sliding component, such as a bearing or a tappet, a portion of the sliding component including a sliding surface is formed of the inventive sintered silicon nitride member, and, in many cases, the sliding surface is polished to a mirror finish. By imparting a lightness or chroma falling within the previously mentioned range to a surface polished to a mirror finish, a previously mentioned defect or foreign matter appearing on the polished surface can be reliably identified. The surface polished to a mirror finish means herein a polished surface having an arithmetical mean roughness Ra of not greater than 0.10 xcexcm. The arithmetical mean roughness Ra as used herein is obtained according to JIS B 0601 (published Feb. 8, 1994, Japanese Standards Association). In measuring surface roughness, a cutoff value and an evaluation length of roughness curve are selected from among those recommended in JIS B 0601 (1994), according to the roughness level to be measured.
The sintered silicon nitride member of the present invention contains a predominant amount of silicon nitride and a balance of a sintering aid component. Such a sintering aid component may be at least one element selected from the group consisting of Mg and elements belonging to Groups 3A, 4A, 5A, 3B (e.g., Al), and 4B (e.g., Si) of the Periodic Table, and may be contained in an amount of 1% by weight to 10% by weight on the basis of oxide. These elements are present within a sintered body in the form of their respective oxides. When the content of the sintering aid component is less than 1% by weight, the sintered body becomes less dense. When the content of the sintering aid component is in excess of 10% by weight, a sintered body suffers lack of strength, toughness, or heat resistance, and a sintered body serving as a sliding component suffers an impairment in wear resistance. Preferably, the sintering aid component is contained in an amount of 2% by weight to 8% by weight.
Examples of a commonly used sintering aid component belonging to Group 3A include Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The content of each of these elements R is expressed on the basis of RO2 for Ce and on the basis of R3O3 for the remaining elements. Particularly, oxides of heavy-rare-earth elements Y, Tb, Dy, Ho, Er, Tm, and Yb are used favorably, since they have the effect of improving strength, toughness, and wear resistance of a sintered silicon nitride body.
The microstructure of the sintered silicon nitride member is such that crystal grains comprising a predominant amount of silicon nitride and forming a main phase are bonded by means of a glassy and/or crystalline binding phase. Preferably, the main phase comprises a predominant Si3N4 phase which, in turn, comprises not less than 70% by volume (preferably not less than 90% by volume) xcex1 phase. In this case, the Si3N4 phase may be such that a portion of Si or N may be substituted by Al or oxygen and may contain metallic atoms, such as Li, Ca, Mg, and Y to form a solid solution. Examples of substituted silicon nitride include sialons represented by the following formulae.
xcex2-sialon: Si6-zAlzOzN8-z(z=0 to 4.2)
xcex1-sialon: Mx(Si,Al)12(O,N)16(x=0 to 2)
M: Li, Mg, Ca, Y, R (R represents rare-earth elements excluding La and Ce)
The aforementioned sintering aid component mainly constitutes the binding phase, but a portion of the sintering aid component may be incorporated into the main phase. The binding phase may contain unavoidable impurities; for example, silicon oxide contained in a material silicon nitride powder, in addition to intentionally added components serving as sintering aids.
Next, a specific method will be described for adjusting the color of at least a surface-layer portion of the sintered silicon nitride member of the present invention so as to fall within the aforementioned range of lightness VS (and/or chroma CS), namely, a method for causing the surface-layer portion to contain a cation of a transition metal element in an amount of 0.1% by weight to 3% by weight. Cations of a transition metal element behave as a color center within many ceramic materials and thus serve as a coloring agent. When the content of cations of a transition metal element is less than 1% by weight, the coloring effect becomes insufficient. When the content of cations of a transition metal element is in excess of 3% by weight, the surface-layer portion is colored to an excessive degree. In either case, the lightness VS of the surface-layer portion falls outside the aforementioned range. Herein, the xe2x80x9csurface-layer portionxe2x80x9d ranges from the surface of the member to a depth of 500 xcexcm (when a state after polishing to a mirror finish is in question, the surface-layer portion ranges from the polished surface to the same depth).
Cations of a transition metal element for use in the present invention are particularly preferably ions of at least one element selected from the group consisting of Ta, Co, Ti, and Fe. Cations of these elements contained in a silicon nitride body to be sintered tend to behave as components which impart to a sintered body a grayish color of low chroma, so that a pore, etc. or foreign matter appearing on the base material serving as background can be more readily identified. Preferably, at least one element selected from the group consisting of Ta, Co, Ti, and Fe is contained in a total amount of 0.1% by weight to 3% by weight on the basis of Ta2O5 for Ta, CoO for Co, TiO2 for Ti, or Fe2O3 for Fe, whereby the aforementioned requirement for lightness VS can be met.
Cations of a transition metal element, such as Ta, Co, Ti, or Fe, serving as a coloring component, may be added in the form of, for example, oxides. When cations of a transition metal element are added in the form of oxides, to a sintered body, a large portion of cations are incorporated into the aforementioned binding phase in the form of oxides, potentially serving as a sintering aid component. However, when cations of a transition metal element are added in an excessive amount, lightness becomes too low, and an impairment in strength of the binding phase results, potentially causing poor toughness and wear resistance of a sintered body.
Cations of a transition metal element serving as a coloring component may be added in any form other than oxide; i.e., cations of a transition metal element may be present in any form other than oxide in a sintered silicon nitride body, so long as they can serve as a color center which develops coloring. For example, cations of a transition metal element may be added in the form of a nitride (for example, TiN). In the case of addition in the form of a nitride, cations of a transition metal element may be present in the form of a nitride in a sintered body or may be converted to an oxide as a result of influence of excess oxygen. In any case, herein, the content of cations of a transition metal element serving as a coloring component is expressed on the basis of an oxide, regardless of the form in which cations are added. Whether or not an element potentially serving as a coloring component, such as Ta, Co, Ti, or Fe, is present in the form of cations can be readily confirmed, for example, by X-ray photoelectron spectroscopy (XPS or ESCA). Specifically, when, in a photoelectronic spectrum obtained by XPS or ESCA, a bond energy peak of an element in question shows a chemical shift in a direction such that ionic valence becomes more positive, the element can be considered to be present in the form of cations.
According to another method for coloring a surface-layer portion of a sintered body, a carbon-thickened layer having a carbon concentration higher than that of an inner-layer portion of the sintered body is formed in the surface-layer portion. In this case, preferably, in order to attain the aforementioned range of lightness VS (3.0 to 9.0), only a trace amount of carbon is present in the carbon-thickened layer. When a large amount of carbon is present in the carbon-thickened layer, the sintered body is colored black, disabling detection of a defect in automatic appearance inspection.
Such a trace amount of carbon can be detected by Raman spectroscopy. In the present invention, in order to attain the aforementioned range of lightness VS, the presence of carbon at a predetermined level must be ensured by Raman spectroscopy. Specifically, in Raman spectroscopy of the surface of a member, a peak ratio represented by X2/X1 is preferably 0.001 to 0.5, where X1 is the intensity of a peak appearing at 206xc2x110 cmxe2x88x921 (a peak corresponding to xcex2-silicon nitride; hereinafter called a xcex2-silicon nitride related peak), and X2 is the intensity of a peak appearing at 1584xc2x120 cmxe2x88x921 (a peak corresponding to graphite; hereinafter called a graphite related peak). When this peak ratio is in excess of 0.2, a sintered body is colored black in its appearance, potentially causing difficulty in detecting foreign matter. When this peak ratio is less than 0.001, a sintered body is colored near white in its appearance, potentially causing difficulty in detecting a pore, etc.
In Raman spectroscopy, observation of simultaneous presence of a silicon nitride related peak and a graphite related peak at a peak ratio X2/X1 (intensity of graphite related peak/intensity of xcex2-silicon nitride related peak) of about 0.5 means that only a trace amount of carbon is present. When a large amount of carbon is present, a sintered body is colored black, disabling detection of a defect in automatic appearance inspection. When the carbon content increases such that the peak ratio X2/X1 is in excess of 0.5, a sintered body is colored near black, causing difficulty in detecting foreign matter. When the peak ratio X2/X1 is less than 0.001, a sintered body is colored near white, causing difficulty in detecting a pore, groove, crack, etc. at the surface. Notably, the intensity of a peak is defined as the height of a peak above a background level. The peak ratio X2/X1 is preferably not greater than 0.2. A carbon component is present in the form of amorphous carbon or polycrystalline graphite. In the case of polycrystalline graphite, in Raman spectroscopy, three peaks appear; at 1350xc2x120 cmxe2x88x921, 1584xc2x120 cmxe2x88x921, and 2710xc2x120 cmxe2x88x921, respectively. In this case, the peak of highest intensity is employed as X2.
According to the present invention, in order to impart a lightness VS of 3.0 to 9.0 to the surface of a sintered body, cations of a transition metal element serving as a coloring component are contained in the sintered body, or a carbon-thickened layer is formed in a surface-layer portion of the sintered body. However, the present invention is not limited thereto. For example, the above range of lightness VS may be attained by adjusting the content of a sintering aid component or by adjusting the firing conditions.